Electrical machines are employed as motors or generators in various technical fields. They usually have a stationary stator as well as a rotationally mounted rotor or impeller and produce waste heat while in operation, which can considerably limit the efficiency and service life of the electrical machine.
Cooling is required in particular for electric machines that have high power density and is realized either with the aid of air or a coolant. Cooling by air requires large surfaces for the effective dissipation of the waste heat produced in the electrical machine, so that the housings must have corresponding dimensions. However, electric drives that are used in a motor vehicle or a rail car for its electrical driving operation, for instance, need to have the most compact design possible in order to optimize the available space. As a result, air cooling is usually not an option for such electric drives.
Cooling jackets through which a fluid is circulated are utilized in order to increase the output and actively cool electrical machines of these types. The currently known cooling approaches for electrical machines are based on radial or axial flows of a fluid coolant through a cooling jacket or a cooling line. The coolant withdraws the waste heat from the electrical machine, so that a fault-free operation is able to be ensured at a high power level. A mixture of water and glycol or oil or some other suitable cooling liquid is frequently used as coolant. The coolant ducts, for example, run parallel to a longitudinal axis of a housing, which in turn lies parallel to a shaft driven by the electrical drive. The coolant ducts, frequently implemented as drill holes, may be distributed across the circumference of the housing, which results in a correspondingly large housing that nevertheless provides only a limited amount of cooling surface. A diversion of the coolant from one coolant duct to another adjacent coolant duct occurs mostly on the face of the electrical machine or of the housing of the electrical machine, in a separate housing part that is connected to the housing. This additionally enlarges the dimensions of the electrical machine. Moreover, expensive sealing between the housing components is required in this case in order to effectively prevent a short circuit caused by coolant that penetrates the housing interior. This is of particular importance when the electrical machine is operated at high powers. An electrical machine used in a motor vehicle is often operated at voltages of up to 800 V and currents approaching 400 A.
An electrical machine that has an annular cooling jacket which radially surrounds the electrical machine is described in German Published Patent Application No. DE 10 2008 040 917 A1, in which liquid coolant is introduced into the cooling jacket via a radially situated coolant inlet and discharged from the cooling jacket again via a coolant outlet that is likewise radially positioned and offset by 180° in relation to the coolant inlet in the circumferential direction of the cooling jacket. Since the coolant inlet or outlet radially projects from the housing, considerable additional space is required for the electrical machine, which is disadvantageous especially given crowded conditions, e.g., in motor vehicles.
Another device for cooling laminated stator cores of electrical machines is described in German Published Patent Application No. DE 10 2007 048 683 A1, in which liquid coolant is introduced into a cooling jacket surrounding the stator, into a plurality of coolant ducts that are evenly distributed along the circumference, and routed out of the cooling jacket in the axial direction by way of two outlets disposed on the end faces of the stator.
German Published Patent Application No. DE 10 2007 011 699 A1, describes a device for heat dissipation of an electrical machine developed as an electromotor, which is provided with a cooling jacket situated between a stator and a housing in the form of a ring. A suitable liquid coolant circulates in the cooling jacket and withdraws the waste heat that is produced during the operation from the electrical machine.
A similar electrical machine, likewise developed as electromotor, is described in German Published Patent Application No. DE 10 2010 023 948 A1, in which to cool the electrical machine, a cooling cavity, which is connected to two ports, is provided and the ports are utilized to supply liquid coolant to the cooling cavity or to remove it from there.
U.S. Patent Application Publication No. 2008/0223557 A1 describes a fluid-cooling system of an electrical machine. The electrical machine has a cooling cavity for the discharge of waste heat, to which coolant is supplied via a radially situated inlet opening and from which coolant is discharged by way of a likewise radially situated outlet opening. Because of the inlet connection, which extends in the radial direction, and the adjacently situated and also radially extending outlet connection at the circumference, more space is required for the electrical machine, which is not available in motor vehicles, in particular.
In this context, example embodiments of the present invention are based on the objective of providing effective cooling for an electrical machine having compact outer dimensions.